During manufacturing of semiconductor devices, such as transistors, quality control monitoring is implemented to ensure that the fabricated devices are conforming to their design specifications. Typically, the device characteristics, e.g., substrate current (I.sub.sub), current drain saturation (I.sub.DSAT), or, current in the linear region (I.sub.DLIN), etc. are monitored and compared to a predetermined value that is used as a standard. This provides a means of detecting problems and/or failures as early in the fabrication process as possible, to minimize further manufacturing costs. Furthermore, these tests provide early warning of reliability problems to the semiconductor device manufacturers and ultimately help to characterize, benchmark and improve the reliability and quality of the semiconductor integrated circuits (ICs) and processes. It is, therefore, desirable that these tests can be conducted both accurately and quickly to provide the fabrication lines with reliable current feedback.
Although different devices generally have different benchmark standards associated with them, there are certain tests that are universally employed by all these devices. For example, tests that help characterize major IC failure mechanisms, which include oxide integrity, electromigration and transistor degradation, are usually utilized regardless of the device type being fabricated.
Transistor degradation characterization is typically defined using a particular device's hot carrier lifetime. As process geometries shrink, electric fields present within semiconductor devices increase, since the distances, across which electrical potentials act, are diminished. The resultant high magnitude electric fields generate hot carriers that are electrons accelerated to relatively high velocities. A hot carrier lifetime is typically a measurement of the length of time it takes for a semiconductor device to degrade an arbitrary amount. Testing involves monitoring the change in a device characteristics under accelerated bias conditions. Failure is defined as the time when a percentage change, e.g., 15%, in the device characteristic under test. A common device characteristic that is used to determine hot carrier lifetime is linear transconductance (g.sub.m). In most applications, the hot carrier lifetime is determined along with the other above-mentioned tests. For example, an initial transconductance measurement, in conjunction with other test measurements, is taken and is then followed by stressing the device and then taking different test measurements again, such as I.sub.DSAT. Subsequent to these measurements, another transconductance measurement is then taken.
Under these present methods, determining the hot carrier lifetime of a device using the degradation of the device transconductance characteristic generally requires either a significant amount of time or a substantially increased voltage above the operating voltage which could be near the breakdown voltage of the device. Depending on the percentage degradation criteria used, e.g., 10% or 15%, the testing (repeated measurement and stress) period may last as long as 100 hours. These long periods of time are, of course, highly undesirable where "real-time" feedback is needed to keep the quality of the device consistently high.
One common approach to reducing the time period is to increase (substantially above the device's operating voltage) the bias, or stress, voltages applied to the device. Using high stress voltages, however, does not truly reflect the conditions seen by the device in common usage. More significantly, the time required to determine the hot carrier lifetime essentially precludes its use during the fabrication process. Instead, current manufacturing determination of a device's hot carrier lifetime involves measuring the device's I.sub.sub characteristic and comparing the measured value with a predetermined value. Unfortunately, however, comparing the value I.sub.sub to a baseline value only provides an approximation, since there is no exact correlation between I.sub.sub and hot carrier lifetime. Furthermore, as a device's process changes, these changes can affect the device I.sub.sub characteristic. In which case, the baseline I.sub.sub standard used in the manufacturing process must also be recalculated. Therefore, as it can be seen from the foregoing, the present methods of determining hot carrier lifetimes of transistor devices lack the accuracy and the short determination times desired in the fabrication process.
Accordingly, what is needed in the art is an improved method for determining a device hot carrier lifetime with greater accuracy and for determining a device hot carrier lifetime that is accomplished in a shorter period of time and uses stress voltages that are closer to the device's operating voltage.